Companionship, which is pretty broad I guess, but also very nice.
To be specific, not just hanging around with anybody, but a single person that is never bothersome when sharing a space with--and by that I mean being around most people is exhausting, but the single person is never a bother in that sense. And we have to be able to be stupid and silly and laugh at everything, but agree on the serious stuff.
tl;dr best friend combined with romantic partner is my favorite thing